Recently, oxide superconducting materials have attracted a great deal of attention. This activity was started by the development of a Ba--La--Cu--O-type oxide superconducting material at IBM Zurich. In addition, yttrium-type oxide superconducting materials are known. It is clear that these have possible applications in solid state electronic devices at liquid nitrogen temperature.
Meanwhile, semiconductor materials containing metals, such as Nb.sub.3 Ge, are also well-known. Attempts have been made to use these metal superconducting materials in solid state electric devices such as Josephson elements.
After well over 10 years of research, Josephson elements containing these metal superconducting materials have almost reached the stage of practical application. However, these superconducting materials have a T.sub.co (temperature at which the electrical resistance becomes zero) at 23.degree. K., which is very low, and so cannot be used without using liquid helium, which is not sufficiently practical.
Meanwhile, since these metal superconducting materials are made entirely of metals, their composition is uniform, both on the surface and in the interior (bulk).
However, when the characteristics of oxide superconducting materials, which have recently attracted attention, are investigated, it is found that their near-surface characteristics (to a depth of about 200 .ANG.) are inferior (less reliable) compared to those in the interior in many cases.
Investigation of the properties of this material has shown that it has a T.sub.co of 90.degree. K. to 100.degree. K., and in addition, the electrical conductivity varies in the range of 150.degree. K. to 270.degree. K.
The cause of this is judged to be that the oxygen in an oxide superconducting material escapes into the air easily when it is near the surface. When the material is heated to 250.degree. C. to 500.degree. C., the oxygen escapes easily even from the interior, causing many oxygen vacancies to be produced. The term of "vacancy" is used to mean an opening where an atom is missing in the regular arrangement of atoms. Whether this oxygen is present in its full amount or insufficient is a critical factor in determining whether the material can be made superconducting or is simply an ordinary electrically conducting material.
This invention enables an oxide superconducting material to be superconducting both in the interior and near the surface, with an optimum density of oxygen vacancies, heat resistance and process resistance which means that the oxide superconducting material can be kept stable even in a vaccum.
Thus, by adding a halogen element to the vacancy, the oxide superconducting material becomes mechanically stronger, so that even if it is made into a thin film, it can be given a T.sub.co of 90.degree. K. or higher. In particular, when it is formed by sputtering, in general it will be formed densely so that it is hard to produce a vacancy, but by adding fluorine to whatever vacancies do form before or at the same time as the thin films is formed, the film that is obtained is dense, heat-resistant and process-resistant.